linux-kernel - Re: [PATCH V3 2/5] cpufreq: ondemand: update sampling rate immediately

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Message-ID: <20151028093109.GJ30039@ubuntu>
Date:	Wed, 28 Oct 2015 15:01:09 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3 2/5] cpufreq: ondemand: update sampling rate
 immediately

On 28-10-15, 07:28, Rafael J. Wysocki wrote:
> Your argument seems to be that it should be OK to do the
> cancel_delayed_work_sync()/gov_queue_work() combo in all cases, because
> even if the new rate is greater than the old one, the user may actually
> want it to take effect immediately and it shouldn't hurt to skip the next
> sample anyway in that case.
> 
> Is this really the case, though?  What about the old rate is 1s, the new one
> is 2s and the timer is just about to expire?  Won't the canceling effectively
> move the next sample 3s away from the previous one which may not be desirable?
> 
> The current code just allows the timer to expire, unless that would prevent
> the new rate from taking effect for too long, which seems perfectly reasonable
> to me.

Okay, what about this case: old rate is 1s, new rate it 5s and we have
just serviced the timer. With the current code we will receive
evaluate again after 1 second instead of 5. Is that desirable ?

I didn't wanted to keep special code for such corner cases. And then
how many times are we going to update sampling rates ?

But if we want to do something special, then we may schedule the work
for following delay:

delay = shared->time_stamp + new_sampling_rate.

shared->time_stamp is the last time we evaluated the load.

With this, we will be at shoot at the exact requested time, relative
to the last time we evaluated the loads.

-- 
viresh
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